1. Field of the invention
The present invention relates to a consumable electrode arc welding machine in which a consumable electrode in the form of a welding wire is automatically fed to a base member which is to be welded and wire droplets are transferred to the base member with a reduced amount of spatter at the time of the welding operation.
2. Description of the prior art
The conventional pulse MIG welding process, in which molten metal from a wire is positively reduced to a spray state for transfer to the base metal by being supplied with a pulse current, has various characteristics different from what is called the ordinary MIG welding process using a constant DC current. One of the great differences between the characteristics of these two welding processes is that when the welding output current is low, the welding operation is performed by repeated short circuits and arcs in the ordinary MIG welding process using DC, while the molten metal from the wire can be transferred to the base metal in the form of spray in the pulse MIG welding process. Transfer in spray form has a distinct feature, as compared with ordinary welding by what might be called shorting transfer, in the generation of spatter. Namely, the welding by spray transfer causes much less spatter than ordinary welding by shorting transfer. This is one of the great advantages of the pulse MIG welding process. In the pulse MIG welding process, however, if the arc voltage is set to a low level and the arc length is reduced, a short circuit is liable to occur between the electrode wire and the base metal. An excessively low voltage setting causes an increased number of shorts, often undesirably leading to as much spatter as the ordinary MIG welding process.
FIG. 1 is a graph showing the amount of spatter plotted against the arc voltage as measured with a conventional pulse MIG welding machine. As shown in FIG. 1, the spatter that is generated sharply increases with a decrease in the arc voltage. The reason for this increase is that the pulse current is supplied during periods set within the welding machine regardless of whether the wire is in contact with the base metal (i.e., the shorted condition) or whether an arc is generated out of contact therewith. That is, the pulse current is supplied regardless of the condition of the weld zone in this respect. This point will be explained in more detail below.
In the conventional pulse MIG welding machine, as described above, a pulse current is supplied during periods set within the welding machine regardless of the conditions of the weld zone. Therefore more spatters are generated by shorting transfer when the arc voltage is set to a low level, that is, when the forward end of the welding wire is in contact with the base metal, thereby increasing the chance that the molten globule at the forward end of the wire will be transferred to the base metal by the pinch effect due to the current or the surface tension of the molten metal. However, when the forward end of the welding wire is out of contact with the base metal, all the molten metal globules at the forward end of the wire are transferred to the base metal in spray form in what is called a spray transfer. The arc voltage may be set to a low level in order to prevent undercutting with an increased welding rate, such undercutting being a welding defect. With the decrease in the welding voltage, however, a great amount of spatter is generated, thereby adversely affecting the advantage of the otherwise small-spatter pulse MIG welding method. The promotion of spatter when the pulse current is applied without regard to the conditions of the weld zone will be explained with reference to FIG. 2.
FIGS. 2a and 2b show changes in the welding current waveform with time against the droplet transfer at the weld zone with time in a conventional pulse MIG welding machine with an arc voltage set to such a low level that shorts occasionally occur. In FIG. 2, reference numeral 9 designates a welding wire, numeral 10 designates a base metal, numeral 91 designates a welding arc, and numeral 92 designates spatter. FIG. 2a represents the case in which the second pulse current is supplied when the welding wire 9 is shortcircuited with the base metal. In this case, the molten metal is released from the short with the dispersion of the spatter 92 at time point t.sub.6 under a strong pinch effect of the pulse current. On the other hand in the case of FIG. 2b where the short is eliminated at time point t.sub.5 immediately before the second application of a current pulse, the second current pulse is applied before the molten drop at the forward end of the welding, wire and the molten metal that has been transferred are joined into needle form, and therefore the pulse current causes spatters 92 at time point t.sub.6.
The foregoing is the description of the prior art relating to the pulse MIG welding process and the reason why spatter increases when the arc voltage is set to a low level in conventional pulse MIG welding machines.